During the semiconductor production process, manufacturers typically test integrated circuit (IC) devices while they are still grouped together as built on a silicon wafer. These tests verify various manufacturing parameters as well as chip function when possible. The connection points or terminals that will eventually be connected to the semiconductor package are typically very small and tightly spaced to conserve space on the wafer and allow for more ICs in the given area. Since the terminals are so small and tightly spaced, specialized tool sets are used to connect the IC to test electronics of a test station.
In basic terms, a precision piece of equipment called a probe station includes a customized precision connection device called a probe card mounted in a manner that allows contact tips of the probe card to be located directly over the terminal pads of the IC device on the silicon wafer. This probe card typically includes contact tips that correspond to the terminals to be contacted and the circuitry on the probe card routes the connection to a test station, which can be a general or special purpose computer.
When the device is powered, the test station looks for specific results and software determines if the IC passes or fails the test. Some applications allow for multiple devices to be contacted and tested at one time, increasing throughput of the testing process. Traditional probe cards consist of a variety of methods to transition the very small and tightly spaced terminal connections on the wafer to more widely spaced connections of the interface to a test station.
One probe style is called a cantilever needle, which is essentially a long precise wire that is shaped and positioned such that the contact tip is located where the terminal on the wafer will be during use to test an electrical device. Groups of needles are assembled into the probe card assembly and each needle is adjusted such that the field of tips all contact the desired terminal positions. Needle probes are typically the least expensive, are well established in the industry, and are widely available.
For mechanical reasons, the needles are relatively long (inches) to provide the required spring properties. Long contact members, however, tend to reduce the electrical performance of the connection by creating a parasitic effect that impacts the signal as it travels through the contact. Resistance in the contacts impacts the self heating effects as current passes through power delivery contacts. Also, the small space between contacts can cause distortion known as cross talk as a nearby contact may influence its neighbor. In most cases, the tests that are run using needle probes are typically basic on-off tests due to the limited signal integrity.
Another probe style is called a buckling beam or vertical probe. Basically, a series of very fine wires can be precisely located within an assembly that locates and guides each wire such that it can be located directly above the IC terminal to be connected. Buckling beam probes have been used for many years and can be based upon IBM technology that is over 20 years old. These probes are usually more expensive, and are typically used for area array connections. Again, the length of the contact wire to allow the beam to buckle is relatively large, so the signal performance can be impacted.
Another probe style is based upon a small precisely formed wire called a micro spring that is created on a sophisticated substrate in such a way that the tip of the wire is in position to contact the desired terminal on the wafer. There are also probe types that are made with semiconductor style processes, creating very small mechanisms commonly referred to as micro-electro-mechanical systems (“MEMS”) devices. Micro spring and MEMS-based probes are typically very expensive, and although better electrically than the longer probe styles, they may have electrical limitations.
Membrane probes are made using a photolithography process to create a very intricate flexible printed circuit member. The precisely shaped probe tips can be routed to the tester electronics through the circuit traces on the flexible printed circuit member. The probe tips are created with a proprietary process where the desired shape can be coined at the desired location into a sacrificial substrate. A series of plating, lithography, sputtering and etching steps are used to create the final circuit.
Membrane probes typically have the best signal performance, but can be expensive to build. For applications requiring higher frequency testing, membrane probes are currently the only choice available. The manufacturing process is very complicated, with many thin film, plating and lithography steps. The complexity of the process results in yield loss and the capital requirements are fairly large. The manufacturing process limits the physical height of the probe tips, such that they do not extend very far off the membrane. There is a potential for the membrane probe to collect debris and crush it into the wafer, thereby damaging ICS. Also, the contact tips must be extremely planar relative to the wafer since the probe tips typically have little or no compliance to compensate for non-planarity.
All of these probe types are rather expensive, costing thousands or even hundreds of thousands of dollars, depending on the type and number of contact points required. Wafer manufactures typically test each device at least once, so durability of the probe tips can be critical.
As processors and electrical systems evolve, increased terminal count, reductions in the terminal pitch (i.e., the distance between the terminals), and signal integrity have been drivers that impact probe tip requirements. As terminal count increases, a certain degree of compliance is required between the contacts on the IC and the probe tips to accommodate the topographic differences and maintain reliable electrical connections. Next generation systems will operate above 5 GHz and beyond and existing probe cards will not achieve acceptable price/performance levels without significant revision.